1. Field of the Invention
This invention relates to a process for the contact drying of aqueous surfactant pastes in a horizontal thin-layer evaporator or dryer.
2. Discussion of Related Art
Anionic and amphoteric or zwitterionic surfactants are important ingredients of solid detergents and bar soaps. The detergents are normally produced by spraying an aqueous, generally highly alkaline slurry of the ingredients and drying the slurry with hot inert gases flowing in countercurrent. However, since this conventional spray drying process is accompanied by serious pollution of the waste air with organic material, there is a need for alternative, ecologically more favorable drying processes. These include in particular the contact drying of water-containing surfactant pastes in thin-layer dryers which leads to dry products which can then be processed with the other dried detergent ingredients, for example in mixers, to form the end product.
European patent application EP-A1 0 572 957 (Kao) describes a process for drying alkyl or alkyl ether sulfates in which dilute surfactant pastes are first concentrated to an active substance content of 60 to 80% by weight and are then dried in vacuo at temperatures of 50 to 140.degree. C. in a vertical thin-layer evaporator. However, a major disadvantage of this process is that, because drying is carried out under reduced pressure, the end product has to be removed from the circuit using complicated equipment suitable for operation in a vacuum. The continuous contact with the hot product means that there is always a danger of caking and, hence, operational disturbances which necessitate a complete stoppage of production so that cleaning can be carried out. Another major disadvantage is that the use of a vertical thin-layer evaporator with wall contact of the rotor blades means that a flowable product film has to be maintained on the wall of the evaporator over its entire length in continuous operation in order to avoid mechanical overloading of the evaporator. Accordingly, the process is not suitable for the direct production of a powder, but only for the production of a concentrated hotmelt which has to be separately crystallized (for example in a flaking roller or the like) and then size-reduced.
By contrast, International patent application WO 96/06916 (Unilever) proposes a process for drying water-containing anionic surfactant pastes in a horizontal thin-layer evaporator which operates under a light vacuum to almost normal pressure and at temperatures above 130.degree. C. Another feature of this process is the use of a very high peripheral speed of the stirrers used of at least 15 m/s which virtually rules out direct wall contact and leads to products of satisfactory color. However, in the drying of water-containing anionic surfactant pastes, more particularly aqueous pastes of alkyl sulfates or alkyl ether sulfates, there is basically a risk of unwanted hydrolysis in the product. Even brief reduction of the pH value leads in the presence of water to rehydrolysis, to the formation of inorganic sulfate and to a reduction in the content of washing-active substance. In following the teaching of WO 96/06916, applicants found that a hydrolysis-free product could not be reproducibly obtained over an operating period of several hours.
Accordingly, the complex problem addressed by the present invention was to provide a process for the contact drying of water-containing anionic surfactant and/or amphoteric surfactant pastes which would not have any of the disadvantages mentioned above and which, despite minimal outlay on equipment, would lead under production conditions to hydrolysis-free, free-flowing granules of satisfactory color distinguished by high bulk densities and a uniform particle size distribution.